Photopolymers are used to make either positive-working or negative-working patterns on various substrates. Solvent-developable photopolymers are a particular class of photopolymers that have the ability to either become more soluble, or less soluble, when exposed to light of a particular wavelength. Positive photopolymers become more soluble, and negative photopolymers become less soluble, in a developer solution after pattern exposure. Thus after exposure, as through a patterned mask, and development of the photoresist with a solvent, a like or opposite pattern of the photoresist remains on the substrate. This developed or patterned photopolymer layer can then be used to prevent the action of a corrosive liquid or etchant (or etch gas when plasma etching is employed) from reacting with, and removing, portions of the substrate not protected by the photopolymer.
In the printing plate art, photoresists are also used to form an image on a substrate, which is generally a metal plate. Ink is applied to the plate having a patterned photoresist layer thereon, and the ink can be transferred to another medium, such as paper for example. This process is well known.
U.S. Pat. No. 5,962,1912 discloses a photoresist for a copper clad aluminum printing plate. A photoresist is applied over the copper layer, and exposed to laser light of a desired frequency only certain areas of the photoresist are exposed to the laser light. The laser light decomposes the azide in those areas exposed to the light; the unexposed regions are then cross linked by flood exposure from a UV lamp, which hardens the photroresist. However, the preferred method is to crosslink the photoresist by UV flood exposure prior to laser exposure. Then the printing plate is developed by solubilizing the laser-exposed regions of the substrate, exposing the underlying copper layer. This exposed copper layer is then etched away. After removal of the exposed copper, a stencil ink is applied to the copper remaining on the aluminum plate mounted on a press, where the pattern is transferred to another medium, such as paper or metal.
The above photoresist then is a positive photoresist, made of an organoazide compound mixed with a suitable film-forming polymer resin, such as polyvinyl formal, and a dye sensitive to the region of the spectrum emitted by the exposing laser. These ingredients are mixed with suitable organic solvents to make a photoresist composition that is sensitive to the laser light, in particular to 830 nm.
Although we do not wish to be bound by any particular theory, we believe the laser light is converted to heat energy by the dye, which is present in an amount sufficient to de-crosslink the photoresist film. The photoresist compositions can be patterned by direct application of a computer controlled laser beam to form finely controlled patterns. Thus these photoresists can be used to transfer patterns directly from a computer, via a computer controlled light beam, to a substrate such as a printing plate.
With the generation of patterns generated by a computer, a laser light source is computer controlled to generate the pattern to a printing plate. The laser scans across the plate, and the computer turns the laser on and off to pattern expose the photoresist. This eliminates the step of making a mask of the desired pattern, and, because the laser light can be finely controlled, an exact amount and frequency of the light exposure of the photoresist, and improved accuracy of the pattern formed, can be obtained.
The metal decorating printing industry uses stainless steel printing plates as the preferred substrate instead of aluminum plates because such plates have a longer life and they are an improvement over aluminum for direct printing of colored patterns on metal, such as for trays, bottle caps, cookie tins and the like. However, the present photoresists are inadequate for stainless steel substrates because they peel off during the processing of the plate. Thus an improved photoresist for decorating metal articles is a desirable commercial advance.